Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 171

TABLE 3.23
Hydraulic Parameters for Modeled Domain
Seepage velocity 111.7 ft/year

Conductivity 0.018 cm/s

Longitudinal dispersivity, α X 26.9 ft

Transverse dispersivity, α Y 2.69 ft

Vertical dispersivity, α Z 0 ft

low-permeability aquifers; however, it is accurate enough for screening-level purposes when
applied to advection-dominated transport conditions (Srinivasan et al., 2007; West et al., 2007;
USEPA, 2007b).
Relative rates of migration of 1,4-dioxane and methyl chloroform in groundwater were deter-
mined by using BIOCHLOR with the i rst-order biological decay coefi cient set to effectively zero
biodegradation for 1,4-dioxane. The objective of the screening-level modeling was to predict rela-
tive rates of migration at release sites and the relative distances within which regulatory thresholds
would be exceeded for 1,4-dioxane and methyl chloroform and its biotransformation products, 1,1-
dichloroethane and chloroethane.
The result of this modeling exercise does not necessarily represent the true i eld behavior of this
mixture of compounds. Among other basic limitations, running BIOCHLOR separately for the
chlorinated ethanes and 1,4-dioxane ignores any competitive sorption that may occur and thereby
possibly underestimates the spatial extent of an actual plume. BIOCHLOR does not account for
aquifer heterogeneities such as channels or other preferential pathways. What is most important is
that BIOCHLOR does not address matrix diffusion in lenses of i ne-grained sediment that occur in
heterogeneous aquifers. This application of BIOCHLOR is not intended to simulate migration
absolutely; rather, it is used to simulate the relative mobility and persistence of 1,4-dioxane in con-
trast to methyl chloroform.
The hydraulic and soil properties of an aquifer studied at Cape Canaveral Air Station, Florida—
included as a preloaded case study in BIOCHLOR—were used to model transport of methyl chlo-
roform and 1,4-dioxane (Table 3.23). With the exception of redei ning the source dimensions,
dispersivities, simulation time, and domain length, all other parameters were left as the defaults of
the Cape Canaveral case study for methyl chloroform simulations. Table 3.24 summarizes the trans-
port parameters for each compound. The model imposes i rst-order decay of methyl chloroform and
its two degradation products, 1,1-dichloroethane and chloroethane, terminating in sequential fash-
ion with ethane. Sorption is modeled according to K oc values. In cases of multiple contaminants, the
median K oc was arbitrarily used.

TABLE 3.24
Regulatory Levels and Transport Properties of Modeled Compounds
TCA

DCA Chloroethane 1,4-Dioxane

Regulatory level (μg/L) 200 5 16 3

K oc (L/kg) 426 130 125 17

Retardation 7.13 2.87 a 2.8 1.1

2.0 1.0 0.7 “0”

Degradation, λ (year −1 )

Notes: K oc values used are BIOCHLOR model defaults for chlorinated ethanes; log K oc = 1.23 for 1,4-dioxane
(17 L/ kg).

a

Retardation value used in model for all chlorinated ethanes.